Why aren't radial engines common for modern piston-powered aircraft?

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Best Answer: I love radials and have several thousand hours flying behind them, but...

Large frontal area = high drag = lower aerodynamic efficiency

High labor requirement and low TBO = high maintenance costs compared to turboprops

Inefficient design = high fuel consumption = high operating cost

High power multi-row radials are highly complex, heavy and inefficient compared to turboprops..

Low compression design doesn't work particularly well with high octane aviation fuel. Lower octane fuel is no longer available.

Aviation fuel for recip engines is expensive and will eventually be phased out for environmental reasons. No suitable substitute has been approved yet so it's not stable market to get into. It's a niche market with limited profitability.

Radial engines have very good power to weight ratios they do have problems presenting such a large front on profile with drag. They have good advantages for fighter aircraft such as those used during WW2 of being easy to build and having no liquid cooling system did not suffer damage as easily as in line water cooled engines did. The inline engines of WW2 made aircraft streamlining much easier so they could get good performance from smaller engines whereas radials had to be much bigger to overcome the drag problems from their large front on profile.
I think a lot of the answer is fuel economy where the more streamlined aircraft with the inline motor can used a smaller motor to get the same speed. Smaller motor means better fuel economy which these days is very important. Back before the 1970s fuel was cheap and fuel economy was not a big concern. But since then fuel economy has become important so inline engines have become the norm. Generally radials are only still in use for planes made before the 1970s, although there us an Australian company called Rotec who make a couple radial engines up to 150hp but these are generally used by aircraft constructors building WW1 replica aircraft to make them look authentic.
For larger aircraft still running propellers now run turboprop engines which give very high power and good efficiency and reliability.

A radial piston engine has to have an odd number of cylinder, and have an acceptable form factor when they have 7 cylinders or 9.
Also, to be efficient, a cylinder has to have an optimum size.
Those things combine to make a "small" radial engine too large and powerful for most small light planes.

And as soon as you reach the realm of power for the typical radial piston engine, you have turbine engine that have the same power and are lighter, on top of using cheap jet fuel instead of expensive avgas.

Most radial engines were replaced by turboprops. The very large R-4360, R-3350, and R-2800engines were directly replaced by larger turboprop engines like the T-34 and 501D (T-56), while the aircraft that used them (the Boeing C-97/377, the Lockheed Constellation, the DC-7 and DC-6) were replaced by turbojet powered aircraft like the KC-135, the 707, the DC-8 ect.

The smaller radials like the R-1820, the R-1830, the R-1340, and the R-985 went on for a bit longer and many are still in service today.. but most have been replaced by smaller turboprop engines like the PT6 and TPE-331s.

At the very low end... opposed engines have proven themselves much superior. Opposed engines are compact and have a low frontal area, they have a simple wet sump oil system, they have a camshaft instead of a complex set of cam rings and planetary gears, as well as a simple crankshaft and con rods that produce even timing vs a massive single throw crank with master and articulating rods which produce uneven timing, and they don't suffer from hydraulic lock or leaking oil from the inverted cylinders.

There is a company out of Australia called Rotec that makes small (>200hp) radials for homebuilts but it is mostly for a nostalgic look and sound... as the aircraft's performance degrades considerably in comparison to other engines.

Poor mileage (gallons per hour), Expensive, complicated, hard to maintain (Some planes had 4 engines in a bank, so back ones ran hot). Heavy for their power output, large frontal area, so hi drag. Hi compression, some require expensive hard to get 104 octane. Ive also heard cold blooded, too.

Radial engines are big, heavy, slow turning engines that burn a lot of fuel and oil. "Modern" aircraft engines like the Continental and Lycoming horizontally opposed four and six cylinder engines are built on technology that was developed more than 80 years ago, and they aren't really that much better than radial engines, but they are lighter weight and it's a bit easier to tuck them into an aerodynamic cowling. More recent Improvements like fuel injection and electronic ignition have improved power and fuel efficiency, but they are still heavy and slow turning engines.

The newest trend in piston powered aircraft engines is liquid cooling and smaller engines that run at higher engine speeds and need reduction gearboxes to slow the prop down to operating speeds.

Aviation in general is NOT a place where you go to find true "cutting edge" technology. People who design and build airplanes, and the people who fly them tend to be very conservative, and are perfectly happy to use the same sort of proven technology that has been around and working well for many years.